[Repo Assist] Set DtdProcessing.Parse explicitly as the default for XML parsing

[github-actions]

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Summary

Sets DtdProcessing = DtdProcessing.Parse explicitly in XmlElement.Create and XmlElement.CreateList in XmlRuntime.fs.

Context

DtdProcessing.Parse is the default on .NET Core (unlike .NET Framework where DtdProcessing.Prohibit is the default). This was the effective behavior before the XXE fix PR (#1596) that was reverted in #1633. Making the setting explicit:

• Documents the intended behavior clearly in code
• Ensures the same behavior regardless of any future framework default changes
• Reflects what dsyme requested: "Make the default DtdProcessing.Parse given that is the default on .NET Core and was the default before the thing we had to revert"

Changes

• Added open System.Xml to imports in XmlRuntime.fs
• XmlElement.Create(reader: TextReader): now uses XmlReader with XmlReaderSettings(DtdProcessing = DtdProcessing.Parse) before loading into XDocument
• XmlElement.CreateList(reader: TextReader): same — uses XmlReader with explicit DtdProcessing.Parse settings

Test Status

✅ Build succeeded
✅ All 69 XML tests passed (dotnet test --filter "Xml")

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[github-actions]

✅ Pull request created: #1636